lubridate error message 'incompatible method's when trying date arithmetic
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                Sharon
            
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        Published on 2013-08-18T13:35:20Z
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            2013/10/22
            3:54 UTC
        
        
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I'm trying to learn lubridate. The lubridate documentation shows:
date <- ceiling_date(date, "month") - days(1)
[1] "2010-05-31 UTC
for date arithmetic. But if I try
mytoday <- now()
first_of_month <- floor_date(mytoday, "month")
first_of_month_last_year <- first_of_month - years(1)
to use date arithmetic to get the first of the month a year earlier I get an error message
Warning message:
Incompatible methods ("-.POSIXt", "Ops.ordered") for "-" 
Any ideas what I'm doing wrong? Thanks.
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